The following relates to the printing arts, reproduction arts, marking device arts, display arts, electrostatic reproduction arts, electrophotographic arts, and related arts. Some illustrative applications of the following include document printing, document photocopying, facsimile printing, and so forth.
Marking devices provide native or machine resolution corresponding to the physical dots or pixels of toner, ink, or other marking material that are marked on a sheet of paper or other media sheet. The marking densities, or the amount of marking material in each physical dot, or similar marking characteristics are impacted by various physical parameters of the marking process. The controls that determine these physical parameters are sometimes referred to as actuators. For electrostatic marking devices, the raster output scanning (ROS) power (e.g., optical power for a scanning laser diode, light emitting diode, or laser diode array), the development field (Vem), and the cleaning field (Vmc) are three parameters that may impact characteristics of the dots or densities marked on media sheets by the marking device.
In halftone marking, a binary “on/off” paradigm is employed for marking pixels. Relatively darker regions have a relatively higher density of marked pixels as compared to lighter regions. In one approach, the pixels of the marked area are delineated into halftone cells, which are also sometimes referred to as “halftone dots”. Each pixel of a halftone cell or dot is assigned a threshold level. Whether a given pixel is “on” or “off” is determined by comparing the threshold level of that halftone pixel with a density level intended for that pixel. The thresholds are selected such that, for a uniform density level across the halftone dot, the fraction of “on” pixels is visually perceived as an average density corresponding to the desired uniform density level. A halftone dot represented by pixel thresholds is referred to as a halftone screen.
The fraction of pixels that are on determines the effective or average density. The distribution of the those pixels within the halftone dot (or, correspondingly, the distribution of thresholds within the halftone screen) can impact the image quality. The thresholds are preferably distributed in the halftone dot so as to minimize banding, moiré patterns, and so forth. Certain spatial threshold patterns, known as “quiet” halftones, are known to facilitate the interpolative modeling of density changes that occur between adjacent quiet halftone levels. The halftone technique is also readily extended to color marking by providing interleaved halftone pixels for cyan, magenta, yellow, and black or for another color blending scheme.
The target or nominal density curve is suitably represented by an array of values, in units appropriate to the sensor, with a discrete value for each printable level. A typical printing system might have 256 (8-bit) or 1024 (10-bit) distinct printable density levels. The thresholds of the halftone screen are selected such that, ideally, a density level represented by a value in the range 0-255 (assuming 8-bit representation), when mapped as a uniform density to halftone and marked by the marking device, appears visually to be at the nominal density value. The relationship between the actual densities provided by the marked halftone dots for uniform patches of the printable density levels is sometimes referred to as the tone reproduction curve (TRC). Ideally, the TRC should be linear with a slope of unity. This gives the minimum step size between any two adjacent levels, resulting in a smooth visual response. This ideal response has also been incorporated in existing standards for offset printing of digital images which are used in customer image processing applications.
In practice, the TRC may vary from this ideal, and may differ from marking device to marking device. The TRC may also drift over time due to changes in humidity, temperature, component wear, replacement of consumable components, toner refill, or so forth. Such drift can cause perceptible, and objectionable color errors in the case of color marking devices.
It is known to adjust the marking device to accommodate machine-specific behavior, and to compensate for drift over time. In one approach, the voltages Vem and Vmc are monitored electrically, e.g. using voltage sensors, and the corresponding actuators are controlled in a closed-loop fashion to maintain these voltages at selected setpoint levels. This approach can ensure good stability, but employs an indirect measure of the TRC and hence may fail to accurately compensate for drift in the TRC.
Another known approach is to measure the actually marked density. For electrostatic printing, this can be done using a density sensor monitoring the toner coverage on the photoreceptor for example using a sensor known as an enhanced toner area coverage (ETAC) sensor, or using a density sensor monitoring the density on the actual paper or other target sheet for example using an inline spectrophotometer (ILS) sensor. Measuring at the photosensor does not consume media sheets and can be performed rapidly on portions of the photoreceptor available during time intervals between processing of sheets. However, toner coverage at the photosensor may not correlate precisely with the actual density marked on the media sheet. Measurements on the media sheet are more accurate, but are slower and consume media sheets.
In one known adjustment process, voltage monitoring is performed on a substantially continuous basis in order to maintain the voltages Vem and Vmc at constant levels. This combats some sources of short-term TRC drift. On an occasional basis, e.g. every few thousand sheets or so, ETAC or ILS measurements are performed, and operational parameters such as ROS power and the setpoints for voltages Vem and Vmc are adjusted. Additionally, in some known approaches the selection of image halftone dots (or, equivalently, the threshold levels of the halftone screen) is also adjusted based on density measurements. These operational parameter adjustments combat TRC drift over time.
The test patches are formed at the various nominal density levels using a standard set of control halftone dots, which ensures fair comparison between density measurements taken at different times. Each control halftone dot is expected to produce a certain predetermined density level on the photosensor or media sheet. Any measured deviation from this predetermined density level from one measurement time to the next indicates a drift or error in the actual TRC exhibited by the installed marking device.
As an added benefit, if the standard set of control halftone dots is predetermined by the marking device manufacturer and installed on every installation of a given marking device model, then the standard set of control halftone dots provide an installation-independent standard for comparison. This enables the performance of a specific installed marking machine to be compared not just with its own performance over time, but also with the performance of other installed marking machines of the same model. This may facilitate diagnosis of a problem with an installed marking device of a standard model.